Warren is an associate professor and director of undergraduate studies in the department of sociology at the University of Minnesota.

According to his analysis, had MPS graduation rates equaled those of MPCP students, there would have been almost 20% more public high-school graduates between 2003 and 2007. Over the five years studied, that would have meant nearly 3,000 additional MPS graduates.

The comparative graduation rate for MPCP students in 2007 was 85%, versus 58% for MPS students.

MPCP is the nation's oldest and largest program to provide public support for parents to enroll their children in private schools. Through the program, 18,500 students this year attend one of 122 private schools in Milwaukee.